const b = () => {
  console.log("b");
  c();
};
const c = () => {
  console.log("c");
  d();
};
const d = () => {
  console.log("d结束了");
};
const a = () => {
  console.log("a");
  b();
};
const main = () => {
  a();
  console.log("main结束");
};
main();
